Patent Documents 1 and 2 listed below disclose hollow poppet valves comprising a valve head integrally formed at one end of a valve stem, the poppet valve formed with an internal cavity that extends from within a valve head into the valve stem and is charged with a coolant together with an inner gas, the coolant having a higher heat conductivity than the valve material. An example of such coolant is metallic sodium, which has a melting point of about 98° C.
Since this type of internal cavity extends from within the valve head into the stem and contains a large amount of coolant, it can advantageously enhance the heat transferability (hereinafter referred to as heat reduction property) of the valve.
If the temperature of a combustion chamber of the engine is heated to an excessively high temperature during an operation, knocking may take place, which lowers the fuel efficiency, and hence the power, of the engine. In order to lower the temperature of the combustion chamber, there has been proposed different types of coolant-charged poppet valve, which is provided with an internal cavity for example filled with a coolant together with an inert gas so as to positively enhance the heat transfer property of the valve.
Conventional internal poppet valves (as disclosed in Patent Documents 1 and 2) comprise a generally disk shape valve head cavity in the valve head and a linear valve stem cavity in the valve stem in communication with the valve head cavity via a smooth interconnect region (that is, a region having a gradually changing inner diameter), so that a coolant charged in the valve head cavity as well as the valve stem cavity and the inert gas can move smoothly between the two cavities, thereby supposedly facilitating an anticipated heat reduction property of the valves.
With this smooth interconnect region, the (liquefied) coolant can move smoothly between the two cavities in response to a valve opening/closing motion of the valve. However, the interconnect region allows upper, middle, and lower layers of the coolant to move in the axial direction of the internal cavity without intermixing the layers.
Consequently, thermal energy stored in lower layers of the coolant (near a heat source) is not positively transferred to middle and upper layers of the coolant, so that the heat reduction property of the valve is not fully fulfilled.
In order to improve the heat reduction property of such hollow poppet valve, Patent Document 3 proposes an improved hollow poppet valve which comprises a diametrically small linear valve stem cavity (formed in a valve stem) connected at a substantially right angle to the circular ceiling of a generally truncated circular cone shape valve head cavity (formed in a valve head) so that, in response to a reciprocal motion of the valve, a smooth flow of coolant is prohibited between the two cavities but instead flows of coolant towards the ceiling past the periphery of the valve head cavity are generated in the valve head cavity, thereby generating vertical circulatory flows of coolant along the axis of the valve (the circulatory flows hereinafter referred to as tumble flows) in the valve head cavity that facilitates positive stirring of the coolant in the valve head cavity and enhances the heat reduction property of the valve (exhibiting a good heat conduction).